The present invention relates to an electronic flash device.
In general, the angle of flash illumination (to be referred to as "the flash-illumination angle" in this specification for brevity) of a flash unit of an electronic flash device used for flash exposures is determined depending upon an angle of view which in turn is dependent upon the focal length of the photographic lens used and the size of the film used. For instance, in the case of a single-lens reflex camera with a standard lens with a focal distance of 50 mm, the flash-illumination angle is about 40.degree.. Therefore, with the electronic flash device designed for use with the standard lens as described above, flash exposures are not adequate with telephoto lenses or wide-angle lenses. To overcome this problem, there have been devised and demonstrated various types of electronic flash devices which may be used with various types of photographic lenses.
In one type, in response to the focal length of a photographic lens used, a reflector which is a factor for determining the flash-illumination angle is changed in configuration, but there arises the problem that the reflector becomes very complex in construction.
In another type, depending upon the focal length of a photographic lens used, a flash lamp, which is a light source, is displaced forwardly or backwardly within a reflector so that the flash-illumination angle may be varied. However, this type has the drawback that a flash unit, which comprises a flash lamp and its reflector, becomes also extremely complex in construction and that when the flash lamp is located too closely to the surface of the reflector, the latter is burned.
In a further type, an optical panel is disposed in front of a flash unit and is moved toward or away from the flash unit depending upon the focal length of a photographic lens used, whereby the flash-illumination angle is varied. However, this type also has the drawback that a mechanism for causing the optical panel to move is complex in construction and subsequently, the electronic flash device becomes large in size.
There has been also well known in the art, an electronic flash device in which a liquid crystal plate or window is used to vary the flash-illumination angle. For instance, a liquid crystal panel is disposed in front of a flash lamp and the angle of light diffusion through the panel is varied by controlling a voltage applied thereto. (See "Applications to Cameras of Liquid Crystals" by Amano, ELECTRONICS (Japanese magazine), FIG. 7, p. 1598, December 1969) However, this reference does not disclose any practical electronic flash device at all. That is, the reference teaches only the underlying principle, but does not teach how to make and use it in practice, thus obtaining advantages of using liquid crystals.